Frequently asked QUESTIONS about DOLBY DIGITAL

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decades of research into how we perceive sound, and delivers 5.1 channels of audio with up to 24-bit precision at an even higher (48 kHz) sample rate than conventional CDs. As a result, Dolby Digital achieves very high sound quality at the low data rates necessary for uncompromised picture quality on such formats as DVD-Video discs and DTV digital television broadcasts. 35. Will Dolby provide 5.1-channel music CDs? Nobody is more enthusiastic <strong>about</strong> multichannel sound than Dolby Laboratories. However, we acknowledge the importance of standardization and compatibility, and the reluctance of producers to release, and retailers to stock, more than one version of a given program. As a result we will not release CDs with Dolby Digital soundtracks, or encourage others to do so. 5.1-channel music CDs are nonstandard; that is, they have to use some form of audio coding other than the PCM audio specified by the CD “Red Book” audio specification. They are also incompatible in that they cannot be heard at all without a special 10 external decoder, and in many cases require a specially equipped CD player as well. Newer formats like DVD-Video discs, on the other hand, have been developed from the outset to support multichannel digital audio. For greater compatibility, these new formats can carry more than one kind of soundtrack. In addition, as explained in question 7, sources with Dolby Digital soundtracks are inherently compatible with mono, stereo, and Dolby Surround Pro Logic playback systems. For older stereo formats, including CD, there’s tried-andtrue Dolby Surround, a fully compatible format enabling twochannel soundtracks to be encoded with four-channel surround sound (see question 6). There are currently more than 700 CD titles encoded with Dolby Surround. When it comes to musiconly recordings with multichannel digital audio, a new kind of DVD disc called DVD- Audio has been developed (see question 36). 36. Can I play the new DVD-Audio discs on my DVD-Video player or my computer’s DVD-ROM drive? Yes…and no. The new DVD-Audio discs can have two zones, one to carry highresolution PCM audio, and the other, which is optional, to carry video programs, such as an artist interview, music videos, or music recordings with still pictures. The audio zone can be played only on the new DVD-Audio players (which typically will also play DVD- Video discs). The optional video zone, however, has the same specifications as DVD-Video discs, so this portion will play on any DVD-Video player and DVD-Video capable ROM drive. This makes it possible for producers to incorporate in the video zone a Dolby Digital version of the same program in the audio zone. If they do so, you will be able to buy DVD- Audio discs as soon as they are available, and enjoy them with Dolby Digital sound until such time you decide to get a new DVD-Audio player to “unlock” their higherresolution audio and other unique program features.
How Dolby Digital Works T he digital audio coding used on compact discs (16-bit PCM) yields a total dynamic range of 96 dB from the loudest sound to the noise floor. This is achieved by taking 16-bit samples 44,100 times per second for each channel, which is often too much data to store or transmit economically, especially when multiple audio channels are required. As a result, new forms of digital audio coding—often known as “perceptual coding”— have been developed that eliminate redundant data, thereby reducing data rates with a minimum of perceived degradation of sound quality. Lessons learned from noise reduction Dolby Digital is the first perceptual coding system designed specifically to process multichannel digital audio. It benefits from Dolby’s development of two prior successful perceptual coding systems, and of what are in essence analog perceptual coding systems: the full gamut of Dolby professional and consumer noise reduction systems. Indeed, Dolby Laboratories’ unique experience with audio noise reduction is essential to Dolby Digital’s data rate reduction, as the fewer the bits used to describe an audio signal, the greater the background noise (known as quantizing noise). Dolby noise reduction works by lowering the noise when no audio signal is present, while allowing strong audio signals to cover or mask the noise at other times. Thus it takes advantage of the psychoacoustic phenomenon known as auditory masking. Even when audio signals are present in some parts of the spectrum, Dolby NR reduces the noise in the other parts so the noise remains imperceptible. This is because audio signals can only mask noise that occurs at nearby frequencies. Putting masking to work Dolby Digital has been designed to take maximum advantage of human auditory masking. It divides the audio spectrum of each channel into narrow frequency bands of different sizes optimized with respect to the frequency selectivity of human hearing. This makes it possible to sharply filter coding noise, so that it is forced to stay very close in frequency to the audio signal being coded. By reducing or eliminating coding noise wherever there are no audio signals to mask it, the sound quality of the original signal can be subjectively preserved. In this key respect, a perceptual coding system like Dolby Digital is essentially a form of selective and powerful noise reduction. With Dolby Digital, bits are distributed among the filter bands as needed by the particular frequency spectrum or dynamic nature of the program. A built-in model of auditory masking allows the coder to alter its frequency selectivity (as well as time resolution) to make sure that a sufficient number of bits is used to describe the audio signal in each band, thus ensuring noise is fully m<strong>asked</strong>. Pooling the bits Dolby Digital also decides how the bits are distributed among the various channels from a common bit pool. This technique allows channels with greater frequency content to demand more data than sparsely occupied channels, for example, or strong sounds in one channel to provide masking for noise in other channels. More bits for audio Dolby Digital’s sophisticated masking model and shared bit pool arrangement are key factors in its extraordinary spectrum efficiency. Furthermore, where other coding systems have to use considerable (and precious) data to carry instructions for their decoders, or to carry the same audio in separate channels for compatibility reasons, Dolby Digital can use more of the transmitted data to represent essential audio, which means inherently higher sound quality. Dolby Digital can process up to 24-bit digital audio signals over a frequency range from 20 Hz to 20 kHz on the full-range channels, and from 20 Hz to 120 Hz on the LFE channel. Sampling rates of 32, 44.1, and 48 kHz are supported at a variety of data rates covering a wide range of requirements. 11
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Frequently asked QUESTIONS about DOLBY DIGITAL

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